Electrical transformers are necessary components in many widely-used energy conversion systems, and, as a result, engineers and scientists are continuously striving to increase the efficiency of these conversion systems. Since the discovery of amorphous metal alloys with Curie-point temperatures above room temperature in the early 1970's, one significant improvement in efficiency has been the use of cores fabricated of extremely thin laminations of amorphous ferromagnetic strips. (See, e.g., U.S. Pat. No. 5,331,304, the disclosure of which is incorporated herein by reference.) The use of amorphous magnetic materials provides improved magnetic characteristics resulting from the inherently lower electrical losses generated by these materials. Because of the absence of a crystal lattice, the magnetic moment in amorphous ferromagnets is not coupled to a particular structural direction, so there is no magneto-crystalline anisotropy. Moreover, since the material is magnetically homogeneous at length scales comparable to the magnetic correlation length, the intrinsic coercivity is small. Consequently, amorphous ferromagnetic cores exhibit soft magnetic behavior characterized by high saturation magnetization, desirable for higher power cores with smaller sizes, low coercivity, low magnetic remanence, and small hysteresis, all of which lead to very low core losses and high efficiencies. Accordingly, amorphous ferromagnetic cores offer improved magnetic coupling characteristics over comparable cores fabricated, for example, of crystalline ferromagnetic alloys.
Amorphous materials are a relatively new class of engineering material, which have a unique combination of high strength, elasticity, corrosion resistance and processability from the molten state. Amorphous materials differ from conventional crystalline alloys in that their atomic structure lacks the typical long-range ordered patterns of the atomic structure of conventional crystalline alloys. Amorphous materials are generally processed and formed by cooling a molten alloy from above the melting temperature of the crystalline phase (or the thermodynamic melting temperature) to below the “glass transition temperature” of the amorphous phase at “sufficiently fast” cooling rates, such that the nucleation and growth of crystals is avoided. As such, the processing methods for amorphous alloys have always been concerned with quantifying the “sufficiently fast cooling rate”, which is also referred to as “critical cooling rate”, to ensure formation of the amorphous phase.
The “critical cooling rates” for early amorphous materials were extremely high, on the order of 106° C./s. As such, conventional casting processes were not suitable for such high cooling rates, and special casting processes such as melt spinning and planar flow casting were developed. Due to the crystallization kinetics of those early alloys being substantially fast, extremely short time (on the order of 10−3 seconds or less) for heat extraction from the molten alloy was required to bypass crystallization, and thus early amorphous alloys were also limited in size in at least one dimension. For example, only very thin foils and ribbons (order of 25 microns in thickness) were successfully produced using these conventional techniques. Because the critical cooling rate requirements for these amorphous alloys severely limited the size of parts made, the use of early amorphous alloys as bulk objects and articles was limited.
Accordingly, conventional amorphous ferromagnetic cores are typically manufactured in continuous strips or ribbons of about 0.001 inch thickness. Cores were produced by concentrically laminating these ribbons around a mandrel forming cores of desired shapes and sizes. Although successful, this process is inherently laborious and expensive. Moreover, cores fabricated of such strips or ribbons usually demonstrate poor fracture toughness. Consequently, such amorphous ferromagnetic cores are subject to easy fracture and great care must be taken in the handling of a core of an electrical transformer fabricated of an amorphous metal in order to minimize undesired fracturing of the amorphous metal laminations of the core. As a result, various arrangements have been proposed for restricting the flexing of the laminations of the amorphous material in order to minimize the fracture mechanism. (See, e.g., U.S. Pat. No. 4,734,975). But fracture is not the only failure mode associated with poor toughness. Fatigue, arising from cyclic loading due to core vibrations during operation, is another failure mode associated with low toughness. Hence, a need exists for amorphous ferromagnetic cores exhibiting higher inherent fracture toughness.
Yet another problem in the fabrication of prior art wound amorphous metal cores is the necessity of maintaining the relative positions of the amorphous metallic strips after winding as closely as possible to their positions. Incorrect replacement of the displaced core ends during the winding procedure can result in large air gaps between the strips and/or significant mechanical stresses within the amorphous metal thereby impairing magnetic performance of the core, and compromising the low core loss characteristics of the amorphous material. Even with careful winding, the packing efficiency of the cores is still lower than the density of the monolithic amorphous metals, which adversely influences the performance of the core in reference to a monolithic core made of a bulk amorphous metal.
Over the years it was determined that the “critical cooling rate” depends strongly on the chemical composition of amorphous alloys. Accordingly, a great deal of research was focused on developing new alloy compositions with much lower critical cooling rates. Examples of these alloys are given in U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; and 5,735,975, each of which is incorporated herein by reference. These amorphous alloy systems, also called bulk-metallic glasses or BMGs, are characterized by critical cooling rates as low as a few ° C./s, which allows the processing and forming of much larger bulk amorphous phase objects than were previously achievable.
In addition to the alloy systems disclosed in the inventions listed above, other inventions were directed to alloy compositions capable of forming bulk ferromagnetic glasses at relatively low cooling rates. For example, US Pat. App. No. 2010/0300148 is directed to bulk amorphous alloys of composition (Fe,Co,Ni)—Mo—(C,B)—(P,Si) that exhibit good glass-forming ability and soft magnetic properties. Additionally, the alloys of that prior art demonstrate high inherent toughness, and thus have the potential to overcome the fracture and fatigue problems encountered in cores fabricated of micrometer-thick amorphous laminae.
Accordingly, a need exists to find a novel approach to fabricate cores from thicker laminae made of bulk ferromagnetic alloys with thicknesses of at least 0.5 mm.